HR (Human Reproduction) Lecture Notes: Placentation
Stages of pregnancy
● Clinical staging
○ First day of last menstrual period (not actually pregnant though)
● 1st trimester, ~12w (embryonic phase)
○ Histiotrophic nutrition (from oviductal and uterine gland
(stimulated by progesterone) secretions)
○ Low pO2, good for stem cells
● 2nd and 3rd trimester (fetal phase)
○ Haemotrophic nutrition (from maternal blood)
○ Need more oxygen and nutrients for developing fetus
Conceptus, outside to inside
● Chorionic villi (lining from trophectoderm)
○ Chorionic villi initially develop all over surface but then regress to
one pole (placenta)
○ Smooth, non-villous tissue is easier to break at birth
● EE coelum (space)
● Yolk sac (like primitive placenta) (ball from hypoblast)
○ By 12d: hypoblast lines trophectoderm around blastocoel ->
makes EE mesoderm -> cavitates to form EE coelum
○ Secondary (definitive) yolk sac: bottom part of primary yolk sac is
pinched off
● Amnion (lining from epiblast)
○ By 12w: expands and obliterates EE coelum
Yolk sac (disappears by 20w, regresses mid-gestation)
● Transfers nutrients from EE coelum to foetus (has microvilli with
transporters such as GLUT1)
● Haematopoeisis
● AFP (Alpha-FetoProtein)
○ Similar to albumin, maintains fluid pressure
○ Marker of fetal health, too much in maternal blood is bad (NTD)
Placenta (mainly (early chorionic villi) developed in 1st trimester)
● Functions
○ Does functions of other organs before they are made
◆ Lung, kidney, gut, most endocrine glands, liver, bone marrow,
immune system
○ Poor function is associated with
◆ IUGR (both low and high birth weight are associated with
neonatal mortality), heart and brain abnormalities, increased
risk of disease
● Structure
○ Discoid, 20-25 cm
, ○ Chorionic plate (faces foetus), basal plate (faces uterus)
○ Lobes containing villi
◆ Intervillous space is small in smokers
○ Villi
◆ Branching (stem -> intermediate -> terminal villi) with more
complexity and functionality
◆ Terminal cells (EVT (ExtraVillous Trophoblast cells)) are
invading into decidua (boundary between uterus and
placenta) to remodel spiral arteries
◆ Initially CTB branches with core of EE mesoderm
◆ Structure
◆ Core of fetal capillaries (branches) and UmA (trunk)
◆ Fetal capillaries are made first (from EE mesoderm
from HB) then join up with artery
◆ Stroma
◆ Bilayer of trophoblast from trophectoderm
◆ Inner CTB (CytoTrophoBlast) that fuse to form outer
(SyncytioTrophoBlast)
◆ STB are trying to find capillaries and glands
◆ Lacunae in STB are precursor intervillus spaces
○ Uterine -> arcuate -> spiral arteries (spiral for expansion)
◆ Squirted at tributaries into centre of lobe, low pressure
◆ Haemochorial (maternal blood in contact with fetal tissue)
◆ Remodelled to reduce pressure to not damage structure of villi
◆ EVTs enter artery -> deposition of fibroid material -> lose
smooth muscle and endothelium -> dilation and loss of
vasoreactivity -> low pressure
◆ Regulated by uNK to restrain invasion
◆ Accreta (into the myometrium), increta (further),
percreta (throughout uterus and maybe into further
tissue such as bladder)
◆ Treated by C section and potentially hysterectomy
◆ Failure can lead to PE, villi damage, poor fetal nutrition
● Further development
○ Villi develop
◆ Microvilli
◆ Intervillus spaces fill with terminal villi
◆ Capillaries move to edge from core (less diffusion distance)
◆ STB nuclei fuse so the rest of the syncytium is thin (less
diffusion distance)
◆ This is also fragile, hence the required low pressure
◆ Synthesis and secrete placental hormones into maternal
circulation
○ SA reaches 12-14 m^2
Stages of pregnancy
● Clinical staging
○ First day of last menstrual period (not actually pregnant though)
● 1st trimester, ~12w (embryonic phase)
○ Histiotrophic nutrition (from oviductal and uterine gland
(stimulated by progesterone) secretions)
○ Low pO2, good for stem cells
● 2nd and 3rd trimester (fetal phase)
○ Haemotrophic nutrition (from maternal blood)
○ Need more oxygen and nutrients for developing fetus
Conceptus, outside to inside
● Chorionic villi (lining from trophectoderm)
○ Chorionic villi initially develop all over surface but then regress to
one pole (placenta)
○ Smooth, non-villous tissue is easier to break at birth
● EE coelum (space)
● Yolk sac (like primitive placenta) (ball from hypoblast)
○ By 12d: hypoblast lines trophectoderm around blastocoel ->
makes EE mesoderm -> cavitates to form EE coelum
○ Secondary (definitive) yolk sac: bottom part of primary yolk sac is
pinched off
● Amnion (lining from epiblast)
○ By 12w: expands and obliterates EE coelum
Yolk sac (disappears by 20w, regresses mid-gestation)
● Transfers nutrients from EE coelum to foetus (has microvilli with
transporters such as GLUT1)
● Haematopoeisis
● AFP (Alpha-FetoProtein)
○ Similar to albumin, maintains fluid pressure
○ Marker of fetal health, too much in maternal blood is bad (NTD)
Placenta (mainly (early chorionic villi) developed in 1st trimester)
● Functions
○ Does functions of other organs before they are made
◆ Lung, kidney, gut, most endocrine glands, liver, bone marrow,
immune system
○ Poor function is associated with
◆ IUGR (both low and high birth weight are associated with
neonatal mortality), heart and brain abnormalities, increased
risk of disease
● Structure
○ Discoid, 20-25 cm
, ○ Chorionic plate (faces foetus), basal plate (faces uterus)
○ Lobes containing villi
◆ Intervillous space is small in smokers
○ Villi
◆ Branching (stem -> intermediate -> terminal villi) with more
complexity and functionality
◆ Terminal cells (EVT (ExtraVillous Trophoblast cells)) are
invading into decidua (boundary between uterus and
placenta) to remodel spiral arteries
◆ Initially CTB branches with core of EE mesoderm
◆ Structure
◆ Core of fetal capillaries (branches) and UmA (trunk)
◆ Fetal capillaries are made first (from EE mesoderm
from HB) then join up with artery
◆ Stroma
◆ Bilayer of trophoblast from trophectoderm
◆ Inner CTB (CytoTrophoBlast) that fuse to form outer
(SyncytioTrophoBlast)
◆ STB are trying to find capillaries and glands
◆ Lacunae in STB are precursor intervillus spaces
○ Uterine -> arcuate -> spiral arteries (spiral for expansion)
◆ Squirted at tributaries into centre of lobe, low pressure
◆ Haemochorial (maternal blood in contact with fetal tissue)
◆ Remodelled to reduce pressure to not damage structure of villi
◆ EVTs enter artery -> deposition of fibroid material -> lose
smooth muscle and endothelium -> dilation and loss of
vasoreactivity -> low pressure
◆ Regulated by uNK to restrain invasion
◆ Accreta (into the myometrium), increta (further),
percreta (throughout uterus and maybe into further
tissue such as bladder)
◆ Treated by C section and potentially hysterectomy
◆ Failure can lead to PE, villi damage, poor fetal nutrition
● Further development
○ Villi develop
◆ Microvilli
◆ Intervillus spaces fill with terminal villi
◆ Capillaries move to edge from core (less diffusion distance)
◆ STB nuclei fuse so the rest of the syncytium is thin (less
diffusion distance)
◆ This is also fragile, hence the required low pressure
◆ Synthesis and secrete placental hormones into maternal
circulation
○ SA reaches 12-14 m^2
